I. Field of the Invention
Our invention relates to means for tying together tubes in superheater platen assemblies, and more particularly, to tie means of the type that are intended to be used to tie together those tubes, which in superheater platen assemblies bear a tangential relationship to each other.
II. Description of the Prior Art
It has long been known in the prior art to provide means operative to effect the tying together of the tubes that superheater platen assemblies embody. Generally speaking, there are at least two principal reasons why it has been found desirable to employ such tie means. First of all, a need to provide support to such tubes has been shown to exist. Secondly, it has been deemed desirable to employ some form of means that would be operative to ensure that the tubes maintain their proper orientation under diverse operating conditions.
As concerns the matter of tube support, under normal operating conditions the tubes are exposed to relatively high temperatures. Such high temperatures give rise to the occurrence of differential expansion of the tubes relative to each other, as well as to the differential expansion of different portions of the same tube. The effect on the tubes of such differential expansion is to subject them to various stresses, which in turn have been known to cause the tubes to fail. Another problem associated with the exposure of the tubes to high temperatures is that of thermal cycling. More specifically, reference is had here to the fact the tubes are often subjected to expansion and contraction on cyclical basis due to changes taking place in the temperatures to which the tubes are being exposed. There have been instances in the past wherein tube failure has been known to have been occasioned by such thermal cycling.
With regard to the question of tube orientation, unless steps are taken to ensure that the desired orientation of the tubes is being properly maintained, the tubes may experience a buildup of stresses that ultimately can lead to one or more tubes failing. For purposes of this discussion, changes in orientation may be construed as encompassing the twisting of one tube relative to another, or the twisting of different portions of the same tube. Or, one tube may experience a longitudinal shift relative to an adjoining tube. Or, the relative orientation of one or moe tubes may be affected in some other fashion. Changes in tube orientation may be occasioned by a variety of different factors such as, for instance, the differential expansion to which the tubes are subjected, or from improper tube installation, etc.
Apart from the ability to provide the tubes with support, and the ability to ensure that the tubes maintain their proper orientation when subjected to operating conditions, there are several other characteristics that it would be advantageous for a tie means to possess. Reference is had here to the fact that the tie means should be susceptible to ease of fabrication as well as to ease of assembly.
There is to be found depicted in the prior art tie means of various types and configurations. For the most part, however, the prior art forms of tie means are designed to be used in those applications wherein the tubes are positioned in spaced relation to each other. Furthermore, most of the tie means that are known to exist in the prior art would be unsuitable, by virtue of the nature of their construction, for use in applications wherein a tangent tube type of construction is being employed, i.e., in situations wherein the tubes bear a tangential relationship to each other.
Although most of the attention on the part of the prior art has heretofore been directed to providing tie means, which would be suitable for use in those applications wherein it is desired that the tubes embodied in a given superheater platen assembly be spaced relative to each other, at least two different forms of tie means are known to exist that are capable of being utilized wherein tangent tube type of construction is contemplated. The two forms of tie means to which reference is had here are the hinge pin tie, and what is commonly referred to in the prior art as a tangent tie weld.
Considering first the nature of the construction of the hinge pin tie, the latter derives its name principally because of its resemblance to a conventional door hinge. One example of such a hinge pin tie can be found illustrated in U.S. Pat. No. 2,834,324--Schoessow. As shown in the latter patent a pair of hinge pin ties are utilized to effect the tying together of a pair of tubes. Each of the pair of hinge pin ties includes a multiplicity of apertured lugs or spacers. In accordance with the preferred embodiment, each hinge pin tie consists of three such apertured lugs with one lug being welded to one of the pair of tubes, and the remaining two lugs being welded to the other one of the pair of tubes. The three lugs are suitably affixed to the pair of tubes so that the single lug is capable of being received in interposed relation between the two lugs that are welded on the same tube. A pin is then inserted through the apertures of the three lugs whereby to effect an interconnection therebetween. The other hinge pin tie is of similar construction, and is assembled in a likewise fashion.
The hinge pin tie form of tie means has met with limited success when it has been employed in certain installations. One major reason for this is that the expansion spaces associated with the apertured lugs have exhibited a tendency to become plugged. Large expansion spaces give rise to overheating of the pin, which in turn can lead to tie failure. In addition, the hinge pin tie has exhibited an inability to successfully resist, i.e., remain free from damage, when the tubes with which the hinge pin tie is being employed are subject to unusual movements that produce high stress patterns.
The tangent tie weld form of construction may be viewed as consisting essentially of the establishment of a solid weld between the two tubes which are intended to bear a tangential relationship to each other. This form of tie means has been found to suffer basically from the same disadvantages as the hinge pin tie. Namely, the tangent tie weld construction has demonstrated a predilection towards functioning in an unsuccessful manner when tube-to-tube temperatures are high, or when the tubes joined by the tangent tie weld undergo unusual movements that give rise to the creation of high stress patterns.
There has thus been shown to exist in the prior art a need for a new and improved form of tie means, which would not suffer from the same disadvantage as those exhibited by prior art forms of tie means, but yet would comprise a tie means that is capable of being utilized to effect the tying together of the tubes, which are to be found embodied in superheater platen assemblies, and more specifically, a tie means that is particularly suited for use in tying together tubes, which bear a tangential relationship to each other. Such a tie means, moreover, should be characterized by its ability to provide the desired degree of support to the tubes, which it serves to join; by its ability to cause the tubes to effectively maintain their proper orientation; by the fact that it is both relatively easy to fabricate and assemble; and by the fact that it is economical both to produce and to employ.